The present invention relates generally to improvements to bar code scanners and bar code scanning. More particularly, the invention relates to advantageous techniques for integrating elements of a detector assembly including a detector used to create a scanner signal based on light entering a scanner and striking the detector as result of reflections of a scan pattern from a bar code, with the light reaching the detector being restricted to a relatively narrow bandwidth.
Bar code scanners often operate by generating a scan pattern produced by reflections of a laser beam from a rotating spinner and from a series of fixed mirrors. The scanner generates a scanner signal based on light that enters the scanner as a result of a reflection of the scan pattern from a bar code. The scan pattern is generated by reflections of a laser beam produced by a scanner laser, with the laser beam produced by the scanner laser being of a particular wavelength.
In order to detect and decode a bar code, the scanner employs a detector including a photodiode or other device that produces the scanner signal upon being struck by the reflections of the scan pattern from the bar code. As noted above, the scan pattern is made up of light produced by reflections of the scanner laser, and this light has a particular wavelength. The wavelength of light to be detected in order to produce the scanner signal is the wavelength of light produced by the scanner laser.
Therefore, it is highly desirable to limit the range of wavelengths of light permitted to strike the detector. It is highly desirable to restrict the wavelength of the light striking the detector to a relatively narrow range that includes the wavelength of light produced by the scanner laser. In addition to reflected light from bar codes, ambient light or light from other sources may enter the scanner and, if allowed to strike the detector, may produce spurious signals and otherwise degrade scanner performance. In order to reduce the effect of extraneous light, the detector is preferably accompanied by one or more optical filters that allow only a narrow range of wavelengths to reach the detector. A typical prior art detector assembly includes a detector comprising detector electronics such as a photodiode and other accompanying electronics. The detector is typically encapsulated in clear plastic. A low pass filter comprising a hot mirror on a glass substrate is affixed to the plastic capsule containing the detector. A hot mirror is an infrared filter that reflects heat energy and admits visible energy. Typical bar code scanners employ lasers producing light having a wavelength in the red region of the visible spectrum, and a hot mirror can be selected that will admit light within this region while also excluding extraneous infrared radiation.
A high pass filter comprising a molded plastic hemispherical lens impregnated with a dye is affixed to the low pass filter. The plastic capsule, the high pass filter and the low pass filter are typically cemented to one another using an index matching epoxy, that is, an epoxy having an index of refraction at or close to the index of refraction of the elements being joined. The use of an index matching epoxy reduces or eliminates reflections that would otherwise be cause by a mismatch between indices of refraction between the epoxy and the elements. The combined effect of the filters is to admit light in a relatively narrow range of wavelengths including the wavelength of the laser used in the scanner, while blocking other ultraviolet, visible and infrared wavelengths that would add unwanted signal and noise effects.
The prior art detector assembly described above presents four optical interfaces to light directed toward the detector. These optical interfaces are the passage from the surroundings to the front of the hemispherical high pass filter, from the high pass filter to the front of the low pass filter, from the front of the low pass filter to the back of the low pass filter and from the back of the low pass filter to the detector surface. Each interface can introduce alignment error and optical loss. In addition, the hemispherical lens is subject to scratching. Moreover, the construction of the detector assembly from several different components introduces cost and complexity to the design. There exists, therefore, a need for a design for a detector assembly for use in a scanner that reduces the number of optical interfaces, that protects the surfaces of the assembly from damage and that allows a relatively simple and low cost construction of the assembly.
A detector assembly according to an aspect of the present invention comprises a molded plastic capsule enclosing a detector. The detector produces a scanner signal when struck by light resulting from the reflection of a scan pattern by a bar code. The plastic capsule is preferably molded around the detector, so that the detector is seamlessly enclosed within the capsule. The capsule includes a hemispherical lens element that is molded as part of the capsule. The plastic from which the capsule is molded is impregnated with a dye chosen to provide filtering characteristics so as to admit a range of wavelengths including the wavelength of light making up the scan pattern.
Once the capsule has been formed, the hemispherical lens element is coated with a coating material chosen to provide filtering characteristics complementary to those of the filtering characteristics of the capsule. The combined filtering characteristics provided by the coating material and the plastic capsule admit light within a range of wavelengths including the wavelength of light making up the scan pattern, and block light having a wavelength above or below the range. The coating is also preferably of a material selected to provide protection against scratches for the hemispherical lens.
A more complete understanding of the present invention, as well as further features and advantages of the invention, will be apparent from the following Detailed Description and the accompanying drawings.